This invention relates to liquid metering apparatus, and is more particularly concerned with a drip metering apparatus capable of producing different drop sizes selectively. The invention is also concerned with a coupling arrangement which may be attached to a drip meter for converting from single to multiple drop-size-producing capability.
The present invention has been devised with a particular view toward overcoming certain disadvantages of conventional parenteral (e.g., intravenous) liquid infusion systems, and accordingly will be described in this context. However, it is to be understood that the basic principles of the invention are not strictly limited to such applications.
In conventional parenteral liquid infusion systems, liquid to be administered to a patient is ordinarily metered through an apparatus comprising a transparent drip chamber which is connected through resilient plastic tubing between a liquid supply and a needle inserted into the patient. The drip chamber is capped by an inlet connector having a single drop-forming opening which forms the liquid received from the supply into drops of a predetermined volume and introduces the drops into the upper region of the drip chamber substantially on the drip chamber axis. The drops fall through the upper region to a reservoir at the bottom of the drip chamber, from which the liquid is conveyed to the patient. The volumetric flow rate to the patient (i.e., the infusion rate), and thus the rate of drop formation, is controlled by a flow regulating device, such as an adjustable clamp mounted on the flexible tubing beyond the drip chamber outlet. By monitoring the rate of drop formation--for example, visually or with a photo-electric drop rate monitor--attending personnel can determine the infusion rate.
The applicability of an infusion system such as just described is determined primarily by its rated drop size. Treatment situations requiring high infusion rates (e.g., resuscitating a patient from shock) or high viscosity liquids (e.g., plasma expanders or blood products) dictate the use of larger drops. Conversely, situations requiring low, precisely controlled infusion rates of lower viscosity liquids (e.g., treatment of pediatric patients or infusion of potent pharmaceutical solutions) dictate the use of smaller drops. In view of these distinctly different requirements, conventional parenteral liquid drip metering equipment is manufactured to produce either large drops (usually about 1/20 ml. per drop) or small drops (usually about 1/60 ml. per drop), and attending medical personnel must select equipment rated appropriately to the task at hand.
The limited applicability of single-drop-size infusion systems presents a number of problems. For example, in emergency situations requiring immediate intravenous access, the initial selection between differently rated infusion sets must often be based on only partial information about the patient's condition. If, as is often the case, further information should prove the initial selection to be inappropriate, the equipment must be changed. This wastes precious time and increases the risk of infection to the patient. Additionally, in emergency and non-emergency situations alike, routine medical procedure or changes in the condition of the patient may require the administration of a liquid incompatible with a previously selected infusion set. Again, the equipment must be changed, wasting time and increasing the risk of infection to the patient.
It is thus apparent that there is a need for a practically designed drip metering apparatus capable of producing multiple drop sizes on a selective basis. Such an apparatus would be advantageous not only in terms of obviating the aforementioned problems, but also in terms of more efficient equipment utilization, by (in many cases) reducing the amount of equipment required to treat individual patients. Moreover, such an apparatus would relieve medical facilities from the burden of maintaining separate inventories of correspondingly rated single-drop-size devices.
In one recent proposal which attempts to address this need, U.S. Pat. No. 4,781,698 ("the '698 patent") discloses a specially designed drip chamber incorporating an adjustable drop forming mechanism. The drip chamber includes relatively rotatable upper and lower sections, the upper section having a discharge opening at its base and the lower section having a plurality of drop tubes mounted at circumferentially spaced eccentric positions on a top plate thereof. The drop tubes, which have different cross-sectional areas for producing different drop sizes, may be brought selectively into registration with the discharge opening by relative rotation of the upper and lower drip chamber sections.
The incorporation of an adjustable drop-forming mechanism into the drip chamber as proposed in the '698 patent creates several problems. First, the manufacturing process becomes more complex and expensive due to the need for multiple castings, moving parts, and precision surfaces sealed by O-rings. Second, the eccentric location of the drop-forming tubes renders the device incompatible with conventional photo-electric drop counters which require that drops be introduced substantially on the axis of the drip chamber. Third, the eccentric geometry renders the device more sensitive to position, as even small deviations from vertical (which are quite common, for example, during transport of emergency patients to the hospital) result in the drops impinging on the side of the drip chamber, making even visual counting difficult.